Bullet proof glass comprises a laminate of generally heat treated annealed 394-472 mils (10-12 mm) glass sheets. For example, four sheets of this special glass are used to form a final laminate. This results in a sheet of glass which is from 1576-1970 mils (40-50 mm) thick. This makes the laminate expensive, difficult to machine and heavy to manipulate.
Other glazed structures for resisting the impact of projectiles are described in U.K. patent 2,098,650 and U.S. Pat. No. 4,774,143 both in the name of General Electric Company. The United Kingdom patent describes a glazed structure having a back laminate comprising a plurality of laminae of polycarbonate, glass or solid resinous materials, with at least one of said laminae being of polycarbonate, and a front laminate comprising a plurality of glass laminae, including a front lamina and a back lamina each of the latter laminae having a thickness of from 30 to 220 mils (0.76 to 5.59 mm) and an intermediate glass core having a thickness greater than either the front or the back lamina. The core in turn may be a single lamina or a plurality of laminae and preferably have a thickness of 220 to 1000 mils (5.59 to 25.4 mm).
The U.S. patent describes a transparent laminated window structure comprising a continuous inner layer of thermoplastic material, and an outer layer substantially parallel to the inner layer including at least a first course of discreet, mutually spaced glass segments, with the space between adjacent segments containing an adhesive material. The window structure may further comprise a strip of elastomeric material compressed between the inner and outer layers so as to define a gap between the layers. As the window structure comprises a number of segments a projectile will only fracture that segment and will not cause cracking in adjacent,.mutually spaced segments in the same course, allowing a high degree of visibility through the window structure upon impact by a projectile. The glass segments are themselves composed of glass laminae making the structure complex, thick and bulky.
Both these prior art specifications describe structures which have a front panel which is a laminate made with several glass laminae, characteristic of "bullet proof glass". Both of these specifications describe the use of an air gap between the front and back laminates of the structures. In the United Kingdom patent the air gap only adds to improve the optical characteristics of the double glazed laminate structure. In the U.S. patent the gas gap is described as not critical to the invention but merely included where the window structure may be exposed to alternating high and low temperature environments which might cause warpage between the front and rear panels of the structure.
Either of these structures would be complex to manufacture, could not be made on site, and would be heavy and expensive.